1. Field of the Invention
The present invention is generally in the field of illumination using electrical energy, and, more particularly, is in the field of illumination utilizing light-emitting diodes (LEDs).
2. Description of the Related Art
Solid-state lighting promises to become the major light source of the near future, not only to save energy but also because the small size and the high efficiency of LEDs promote far more versatility of applications than did previous lighting technology. This in turn has stimulated the field of illumination optics, which previously was nearly all reflective, with lenses confined only to such expensive niche applications as stage lighting. Of the numerous illumination applications that have recently adopted LEDs as the light sources, many have lenses specifically designed for them. The lenses are inexpensive plastic devices, which are much easier to integrate than glass parts. A majority of these illumination applications are circularly symmetric, which makes both design and mold-fabrication much easier, and best matches the hemispherical output geometry of LEDs. The prevalence of circular output-beams means that rectangular zones of desired illumination may not be properly served. Previous patents, such as, for example, U.S. Pat. Nos. 5,924,788 and 6,273,596 to Parkyn, Jr., and U.S. Pat. No. 7,042,655 to Sun, Falicoff and Parkyn, Jr. address this need in similar ways, having in common the basic method of corresponding flux-tessellations utilizing identical grids with cells of identical flux but variable size laid out for both source and output beam on the unit sphere of directions. Subsequent advances by an inventor herein have applied recent mathematical advances in computer graphics to originate a new and improved way to design illumination lenses that are not circularly symmetric. In general, surfaces that are not rotationally symmetric have come to be encompassed by the term “free-form.”
The embodiments and design methods disclosed herein are encompassed by the field of nonimaging optics. The general problem addressed herein is how to design free-form surfaces that transform the intensity distribution of a point source (i.e., a 1 millimeter LED relative to a 1 inch lens) into a uniform illumination distribution on a target. In particular, the problem of rectangular targets has been neglected in the prior art except for the above-referenced patents of the Inventor. The general problem of transforming the intensity distribution of a source is exactly posed in abstruse mathematical terms by Roland Winston et al., in Freeform Optical Designs for Point Sources in 3D, Nonimaging Optics, Section 7.7, Elsevier Academic Press, 2006, pages 173-178, which is hereby incorporated by reference herein. No examples are given, however, of any actual application of these formulae.
A free-form surface is the solution of a constrained three-dimensional partial differential equation, a subject without much mathematical literature. Only in the image-processing specialty of shape-from-shading is the subject of surface synthesis explored in any detail. In that field, however, the problem is the recovery of terrain or surface detail that is known already to exist, so that a mathematical solution is guaranteed and rectangular geometry is inherent from the outset. In free-form lens design, however, solutions are not guaranteed and the basic geometry is spherical. In summary, the field of the invention is relatively undeveloped, particularly so for illuminating rectangular zones uniformly.